Therapeutic and Diagnostic NP Platforms and Nanovectors.
Nanoscience is being developed in conjunction with advanced medical science for further precision in diagnosis and treatment. Multidisciplinary biomedical scientific teams including biologists, physicians, mathematicians, engineers and clinicians are working to gather information about the physical properties of intracellular structures upon which biology's molecular machines are built. A new emphasis is being given to moving medical science from laboratory to the bedside and the community. This platform development program brings together an outstanding laboratory that is pioneering biomedical applications of PAA nanovectors (Kopelman), together with an innovative porphyrin chemistry and a world-class PDT group at RPCI that is highly experienced in the high volume screening and in vitro/in vivo evaluation of novel compounds, and in developing new therapies from the test tube to FDA approval for clinical use. Although nanoplatforms and nanovectors (i.e. a nanoplatform that delivers a therapeutic or imaging agent) for biomedical applications are still evolving, they show enormous promise for cancer diagnosis and therapy. The approach has been the subject of several recent reviews2 Therapeutic examples include NP containing PDT agents, folate receptor-targeted, boron containing dendrimers for neutron capture and NP-directed thermal therapy. Recently, we have evaluated the therapeutic and imaging potential of encapsulated, post-loaded and covalently linked photosensitizer-NPs. In PAA NP the post-loading efficiency showed enhanced in vitro/in vivo therapeutic and imaging potential. PAA NP have core matrixes that can readily incorporate molecular or small NP payloads, and can be prepared in 10-150 nm sizes, with good control of size distributions. The surfaces of NPs can be readily functionalized, to permit attachment of targeting ligands, and both are stable to singlet oxygen (1O2) produced during PDT. PAA-NP have the advantages of (1) A relatively large knowledge base on cancer imaging, PDT, chemical sensing, stability and biodegradation. (2) No known in-vivo toxicity. (3) Long plasma circulation time without surface modification (see Preliminary Data), but with biodegradation and bioelimination rates controllable via the type and amount of selective cross-linking (introduced during polymerization inside reverse micelles). (4) Scale-up to 400 g material has been demonstrated, as well as storage stability over extended periods. Limitations include relative difficulty in incorporating hydrophobic compounds (although we have accomplished this), leaching of small hydrophilic components unless they are “anchored”, and unknown limitation on bulk tumor permeability because of hydrogel swelling.
PDT and Cancer Therapy.
The major challenge of cancer therapy is preferential destruction of malignant cells with sparing of the normal tissue. Critical for successful eradication of malignant disease are early detection and selective ablation of the malignancy. PDT is a clinically effective and still evolving locally selective therapy for cancers. The utility of PDT has been demonstrated with various photosensitizers for multiple types of disease. It is FDA approved for early and late stage lung cancer, obstructive esophageal cancer, high-grade dysplasia associated with Barrett's esophagus, age-related macular degeneration and actinic keratoses. PDT employs tumor localizing PSs that produce reactive 1O2 upon absorption of light which is responsible for the destruction of the tumor. Subsequent oxidation-reduction reactions also can produce superoxide anions, hydrogen peroxide and hydroxyl radicals which contribute to tumor ablation4. Photosensitizers have been designed which localize relatively specifically to certain subcellular structures such as mitochondria, which are highly sensitive targets5. On the tumor tissue level, direct photodynamic tumor cell kill, destruction of the tumor supporting vasculature and possibly activation of the innate and adaptive anti-tumor immune system interact to destroy the malignant tissue6. The preferential killing of the targeted cells (e.g. tumor), rather than adjacent normal tissues, is essential for PDT, and the preferential target damage achieved in clinical applications is a major driving force behind the use of the modality. The success of PDT relies on development of tumor-avid molecules that are preferentially retained in malignant cells but cleared from normal tissues. Clinical PDT initially was developed at Roswell Park Cancer Institute (RPCI), and we have one of the world's largest basic and clinical research programs. The RPCI group developed Photofrin®, the first generation FDA approved hematoporphyrin-based compound. Subsequently, our group has investigated structure activity relationships for tumor selectivity and photosensitizing efficacy, and used the information to design new PSs with high selectivity and desirable pharmacokinetics. Although the mechanism of porphyrin retention by tumors in not well understood, the balance between lipophilicity and hydrophilicity is recognized as an important factor7 In our efforts to develop effective photosensitizers with the required photophysical characteristics, we used chlorophyll-a and bacteriochlorophyll-a as the substrates. Extensive QSAR studies on a series of the alkyl ether derivatives of pyropheophorbide-a (660 nm) led to selection of the best candidate, HPPH (hexyl ether derivative) 8,9, now in promising Phase II clinical trials. Our PS development now extends to purpurinimide (700 nm) and bacteriopurpurinimde (780-800 nm) series with high 102 producing capability10-13 Long wavelength absorption is important for treating large deep-seated tumors, because longer wavelength light increases penetration and minimizes the number of optical fibers needed for light delivery within the tumor
Advantages of Longer Wavelength Photosensitizers (700-800 nm) for Phototherapy Over HPPH:
The penetration of light through tissue increases as its wavelength increases between 630 and 800 nm. Once light has penetrated tissue more than 2-3 mm it becomes fully diffuse (i.e. non-directional). In diffusion theory, the probability that a photon will penetrate a given distance into tissue is governed by the probability per unit path. The intrinsic absorption of most tissues is dominated by hemoglobin and deoxyhemoglobin, with the strong peaks of the absorption bands at wavelengths shorter than 630 nm. The tails of these bands extend beyond 630 nm and grow weaker with increasing wavelength. Thus the probability of a photon being absorbed by endogenous chromophores decreases with increasing wavelength from 630-800 nm and the scattering also decreases with wavelength14 resulting in the very large increase in light penetration at ˜600 to 800 nm.
PDT and Nanoparticle Platforms
Photosensitizers have several very desirable properties as therapeutic agents deliverable by NP: (1) Only a very small fraction of administered targeted drug makes it to tumor sites and the remainder can cause systemic toxicity. However, PDT provides dual selectivity in that the PS is inactive in the absence of light and is innocuous without photoactivation. Thus the PS contained by the NP can be locally activated at the site of disease. (2) PDT effects are due to production of 1O2, which can readily diffuse from the pores of the NP (see Preliminary Data). Thus, in contrast to chemotherapeutic agents, release of encapsulated drug from the NP, is not necessary. Instead, stable NP with long plasma residence times can be used, which increases the amount of drug delivered to the tumors. (3) PDT is effective regardless of the intracellular location of the PS. While mitochondria are a principal target of 1O2, PS incorporated in lysosomes are also active the photodynamic process causes rupture of the lysosomes with release of proteolytic enzymes and redistribution of the PS within the cytoplasm. NP platforms also provide significant advantages for PDT: (1) High levels of imaging agents can be combined with the PS in the NP permitting a “see and treat” approach, with fluorescence imageguided placement of optical fibers to direct the photoactivating light to large or subsurface tumors, or to early non clinically evident disease. (2) It is possible to add targeting moieties, such as cRGD or F3 peptide to the NP so as to increase the selective delivery of the PS. (3) The NP can carry large numbers of PS, and their surface can be modified to provide the desired hydrophilicity for optimal plasma pharmacokinetics. Thus, they can deliver high levels of PS to tumors, reducing the amount of light necessary for tumor cure.
Molecular Targeting.
F3/Nucleolin Targeting
F3 peptide is a 31-amino acid synthetic peptide derived from a fragment of the nuclear protein, high mobility group protein 2 (HMGN2)15. HMGN2 is a highly conserved nucleosomal protein thought to be involved in unfolding higher-order chromatin structure and facilitating the transcriptional activation of mammalian genes 62 when injected i.v., F3 peptide internalizes and accumulates in the nuclei of HL-60 cells and human MDA-MB-35 breast cancer cells. Tissue and cellular localization of F3 peptide indicated that it homes selectively to tumor blood vessels and tumor cells and has the remarkable property of being able to carry a payload into the cytoplasm and nucleus of the target cells. Furthermore, NPs with surface attached F3 behave similarly, attaching selectively to nucleolin expressing cells, and then channeled towards the cell nucleus. Recent literature shows that the F3 peptide binds to cell surface-expressed nucleolin on the target cells. Although primarily known as a nuclear and cytoplasmic protein a cell surface form of nucleolin also exists. Nucleolin is expressed on the surface of MDA-MB-35 cells1 and shuttles between the cytoplasm and the nucleus and between the cell surface and the nucleus. Nucleolin is also overexpressed in 9L glioma cells. Therefore, the mechanism of F3 targeting is recognition by nucleolin at the surface of actively growing cells (tumor cells and neovascular endothelial cells), which then binds and internalizes it, and transports it into the nucleus. While nucleolin can carry F3-targeted molecules from the cell surface into the nucleus, F3-labelled PAA nanoparticles containing Photofrin accumulated in the cytoplasm, which is useful because mitochondria are the primary target of PDT-produced 1O2. F3 targeting has been used recently to deliver nano-sized particles composed of lipids or quantum dots to tumor vasculature.
Integrin Targeting.
Integrins are a major group of cell membrane receptors with both adhesive and signaling functions. They influence behavior of neoplastic cells by their interaction with the surrounding extracellular matrix, participating in tumor development16. Integrin αVβ3 in tumor cells binds to matrix metalloprotease-2 in a proteolytically active form and facilitates cell-mediated collagen degradation and invasion. It over-expresses in U87 and 9L glioma tumors. An increase in its expression is correlated with increased malignancy in melanomas. αvβ3 plays a critical role in angiogenesis and is up-regulated in vascular cells within human tumors. Significant overexpression of αvβ3 is reported in colon, lung, pancreas, brain and breast carcinomas, which was significantly higher in metastatic tumors. Our objective is to prepare a known integrin αvβ3-targeting ligand. While some recent work suggests that dimeric RGD peptides provide additional affinity and tumor binding, our recent in vitro data with HPPH-RGD conjugates (in one of which the binding site was blocked) shows the validity of our approach using monomeric RGD peptides.
Imaging
Optical Imaging and Tumor Detection.
Multiple, complementary techniques for tumor detection, including magnetic resonance, scintigraphic and optical imaging are under active development. Each approach has particular strengths and advantages. Optical imaging includes measurement of absorption of endogenous molecules (e.g. hemoglobin) or administered dyes, detection of bioluminescence in preclinical models, and detection of fluorescence from endogenous fluorophores or from targeted exogenous molecules. Fluorescence, the mission of absorbed light at a longer wavelength, can be highly sensitive: a typical cyanine dye with a lifetime of 0.6 nsec can emit up to 1032 photons/second/mole. A sensitive optical detector can image <103 photons/second. Thus even with low excitation power, low levels of fluorescent molecular beacons can be detected. A challenge is to deliver the dyes selectively and in high enough concentration to detect small tumors. Use of ICG alone to image hypervascular or “leaky” angiogenic vessels around tumors has been disappointing, due to its limited intrinsic tumor selectivity. Multiple approaches have been employed to improve optical probelocalization, including administering it in a quenched form that is activated within tumors, or coupling it to antibodies or small molecules such as receptor ligands. Recent studies have focused on developing dye conjugates of small bioactive molecules, to improve rapid diffusion to target tissue and use combinatorial and high throughput strategies to identify, optimize, and enhance in vivo stability of the new probes. Some peptide analogs of ICG derivatives have moderate tumor specificity and are entering pre-clinical studies. However, none of these compounds are designed for both tumor detection and therapy. It is important to develop targeting strategies that cope with the heterogeneity of tumors in vivo, where there are inconsistent and varying expressions of targetable sites.
Photosensitizers are not Optimal for Tumor Detection
Photosensitizers (PS) generally fluoresce and their fluorescence properties in vivo has been exploited for the detection of early-stage cancers in the lung, bladder and other sites 17 For treatment of early disease or for deep seated tumors the fluorescence can be used to guide the activating light. However, PS are not optimal fluorophores for tumor detection for several reasons: (i) They have low fluorescence quantum yields (especially the long wavelength photosensitizers related to bacteriochlorins). Efficient PS tend to have lower fluorescence efficiency (quantum yield) than compounds designed to be fluorophores, such as cyanine dyes because the excited singlet state energy emitted as fluorescence is instead transferred to the triplet state and then to molecular oxygen. (ii) They have small Stokes shifts. Porphyrin-based PS have a relatively small difference between the long wavelength absorption band and the fluorescence wavelength (Stokes shift), which makes it technically difficult to separate the fluorescence from the excitation wavelength. (iii) Most PS have relatively short fluorescent wavelengths, <800 nm, which are not optimal for detection deep in tissues.
Advantages and Limitations of Bifunctional Photosensitizes Fluorophore Conjugates.
In a separate study we have developed certain bifunctional conjugates that use tumor-avid PS to target the NIR fluorophores to the tumor18. The function of the fluorophore is to visualize the tumor location and treatment site. The presence of the PS allows subsequent tumor ablation. The optical imaging allows the clinician performing PDT to continuously acquire and display patient data in real-time. This “see and treat” approach may determine where to treat superficial carcinomas and how to reach deep-seated tumors in sites such as the breast, lung and brain with optical fibers delivering the photo-activating light. A similar approach was also used for developing potential PDT/MRI conjugates in which HPPH was conjugated with Gd(III)DTPA Due to a significant difference between imaging and therapeutic doses, the use of a single molecule that includes both modalities is problematic. However, with PAA NPs we were able to solve this problem.
PS-Directed PET Imaging.
Positron emission tomography (PET) is a technique that permits non-invasive use of radioisotope labeled molecular imaging probes to image and assay biochemical processes at the level of cellular function in living subjects20. PET predominately has been used as a metabolic marker, without specific targeting to malignancies. Recently, there has been growing use of radiolabeled peptide ligands to target malignancies. Currently, PET is important in clinical care and is a critical component in biomedical research, supporting a wide range of applications, including studies of gene expression, perfusion, metabolism and substrate utilization, neurotransmitters, neural activation and plasticity, receptors and antibodies, stem cell trafficking, tumor hypoxia, apoptosis and angiogenesis21. Available isotope labels include 11C (t1/2=20.4 min), 18F (t1/2=110 min), 4Cu (t1/2=12.8 h) and 124I (t1/2=4.2 days). For targeting, a long circulation time may be desirable, as it can increase delivery of the agent into tumors. HPPH and the iodobenzyl pheophorbide-a have plasma half lives ˜25 h. The long radiological half life of 124I is well matched to the pheophorbides; it permits sequential imaging with time for clearance from normal tissue. Labeling techniques with radioiodine are well defined with good yield and radiochemical purity22. Despite the complex decay scheme of 124I which results in only 25% abundance of positron (compared with 100% positron emission of 18F), in vivo quantitative imaging with 124I labeled antibodies has been successfully carried out under realistic conditions using a PET/CT scanner A variety of biomolecules have been labeled with 124I. We have devised a coupling reaction which rapidly and efficiently links 124I to a tumor-avid PS23-25, and used the conjugate to target and image murine breast tumor and its metastasis to lung (See Experimental Section). Acquisition of clinical PET images can be slow, but combination PET-CT scanners allow real time guidance of therapeutic interventions. Also, new developments in tracking may permit real time interventions guided by PET data sets.
NPs can Optimize Tumor Detection and Treatment of Brain Tumors:
A photosensitizer (PS) with increased selectivity and longer wavelength could be a more suitable candidate for brain and deeply seated tumors (especially breast, brain and lung). The evolution of light sources and delivery systems is also critical to the progression of photodynamic therapy (PDT) in the medical field. Two different techniques: interstitial and intracavitary light delivery have been used for treatment of brain tumors. Powers et al26 using interstitial PDT on patients with recurrent brain tumors showed that the majority of patients had tumor recurrence within two months of treatment. However, it was later observed that treatment failures appeared to occur outside the region of the effective light treatment. Chang et al reported an effective radius of tumor cell kill in 22 glioma patients of 8 mm compared with the 1.5 cm depth of necrosis noted by Pierria with the intracavitary illumination method. It is believed that tumor resection is important so that the numbers of tumor cells remaining to treat are minimized. With stereotactic implantation of fibers for interstitial PDT there is no cavity to accommodate swelling and a considerable volume of necrotic tumor which causes cerebral edema. However, cerebral edema can be readily controlled with steroid therapy. Compared to chemotherapy and radiotherapy, patients with brain tumors treated with PDT have definitely shown long-term survival, whereas glioma patients treated with adjuvant chemotherapy or radiotherapy do not show additional benefits as reported by Kostron et al27 and Kaye et al.28 On the basis of our preliminary data, the αvβ3 targeted NPs may improve tumor-selectivity and PDT outcome.
Importance of Multifunctional NPs in Brain-Tumor Imaging and PDT:
The prognosis for patients with malignant brain tumors is linked to the completeness of tumor removal. However, the borders of tumors are often indistinguishable from surrounding brain tissue so tumor excision is highly dependent upon the neurosurgeon's judgment. To identify tumors, neurosurgeons use diagnosticimaging methods such as Computed Tomography (CT) or Magnetic Resonance Imaging (MRI), which enhance the contrast between tumor and surrounding brain tissue. However, there are frequently discrepancies between intraoperative observations of tumor margins and preoperative diagnostic imaging studies. Unlike CT and MRI, intraoperative ultrasound can provide real-time information to locate the tumor and define its volume. However, once resection commences is also limited by signal artifacts caused by blood and surgical trauma limit tumor identification at the resection margin. Intraoperative MRI allows the neurosurgeon to obtain images during surgery, which can improve the completeness of the tumor resection, however microscopic disease is still not detected. In an ideal situation, the surgeon would perform the brain tumor resection with continuous guidance from high-contrast fluorescence from the tumor observed directly in the resection cavity.